There are three distinct parts in the project. First, we are going to test the experimental basis of the equilibrium-point hypothesis and to compare some of the predictions of the hypothesis with experimental observations. In particular, the reproducibility of performance of subjects during different single-joint motor tasks requiring "not to intervene voluntarily" will be studied, the performance under instructions "not to intervene" and "to intervene" will be compared, electromyograms during perturbed single-joint movements will be theoretically predicted and compared with experimental observations. The second part of the project will be based on a new method, recently designed in our laboratory, which enables reconstruction of joint compliant characteristics and central control variable changes ("virtual trajectories") during single-joint movements. The method will be used for analysis of discrete single-joint movements and oscillatory movements against different loads. Reconstructed patterns of control variables will be compared with those predicted from the equilibrium-point hypothesis. Two hypotheses will be tested: 1). Joint stiffness changes are directed towards minimizing the difference between movement frequency and limb natural frequency; and 2) At the limb natural frequency, the virtual trajectory will have minimal peak-to-peak changes while the electromyograms of the major muscle groups will change monotonically with frequency. During the third part of the project, we plan to study virtual trajectories of multi-joint movements. In particular, we will be interested in compliant properties of the working point and individual joints and in compensation of errors created in one of the joints by corrections in other joints. The major hypothesis to be, tested is: The control variable for multi-joint movements is three-dimensional compliance of the working point. Experiments will be performed in which the subjects will learn a standard movement against a constant external load. Then, they will be required to reproduce the learned motor program while changes in the external load will occur in some of the trials. The subjects will be instructed "to reproduce the same time pattern of motor command while ignoring possible external load changes". Kinematic and dynamic parameters of the movements will be recorded. Electromyograms of major muscle groups will be recorded as well.